GIT_ALLOC_LIMIT limits xmalloc()'s size, which is of type size_t.
Better use git_env_ulong() to parse the environment variable, so that
the postfixes 'k', 'm', and 'g' can be used; and use size_t to store the
limit for consistency. The change to size_t has no direct practical
impact, because we use GIT_ALLOC_LIMIT to test small sizes.